rm exclusion_criteria_checked

This field was meant to covere two aspects:

1.  Remind users to check exclusion criteria.
2.  Document that exclusion criteria have already been checked in an
    earlier call/mail.

(1) is not really that relevant because recruiters usually know the
criteria by heart after a few calls. It is nice to have the text
available, but it is usually not necessary for each participant.

(2) also has limited utility. For phone recruitment, everything is
usually checked in a single call. For mail recruitment there are usually
more than one mail, but those have a relatively clear structure.
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